Human Anatomy and Physiology

The Digestive System

The Digestive System - Function

• Dissembles food to small molecules that the body can absorb.

• Components: – Mouth, pharynx, esophagus stomach, small

intestine, large intestine

– Accessory structures: Salivary glands, pancreas, liver. These add secretions

Digestive System

• Total length 8-9 meters– 2/3 small intestines

Mouth

Salivary glands

Stomach

Pancreas (behind stomach)

Large intestine

Small intestine

Rectum

Gallbladder (behind liver)

Liver

Esophagus

Pharynx

The digestive system

Upper Digestive System

• Lips, Mouth (lips, tongue, teeth), salivary glands, pharynx, esophogus,

Mouth

• Mechanical and Chemical Digestion– Mastication – “chewing”

• Lips – hold food in• Tongue – moves food around• Teeth – cut, tear and crush

– Salivation – chemical digestion begins• Moistens food – salt, water, mucous• Amylase – starch breakdown to sugars• Lysozyme – breaks down cells (especially pathogens)

• End result: Bolus– Bolus pushed to esophagus– Tongue and throat muscles

Salivary Glands and Pharynx

• Salivary glands– Three pairs

• Parotid• Submandibular• Sublingual

• Pharynx• Common passage

• Epiglottis• Covering for the glottis (opening to the larynx)

Esophogus – about 25 cm (10 inches)

• Peristalsis– Movements in the “food tube” that push food to the

stomach (voluntary muscle)• Can eat in no gravity• Can eat lying down

• Cardiac Sphincter (involuntary)– Ring of muscle between stomach and esophagus– Heartburn; acid reflux

• Overeating, high acid foods, high caffeine

Stomach

• Muscular sac• Chemical Digestion

• Mucus – lubricates and protects the stomach wall• Hydrochloric acid – provides acidity• Pepsin

– Activated by HCl– Begins protein digestion (with HCl)

• Muscular Digestion– Fluids and food mix to form chyme– After ~1 to several hours, goes through pyloric

valve to duodenum (1st part of small intestine)

Ulcers

• Caused by Helicobacter pylori– Requires antibiotics, not bland diets– Bacteria damage cells of stomach lining, make

stomach susceptible to ulcers, acid further irritates.

Duodenum

• 1st part of small intestine• Most chemical digestion here

– Enzymes & digestive juices from pancreas, liver, and duodenum lining

Digestive System

Site

Mouth

Stomach

Small intestine(from pancreas)

Small intestine

Site

Mouth

Stomach

Small intestine(from pancreas)

Small intestine

EnzymeEnzyme Role in DigestionRole in Digestion

Salivary amylase

Pepsin

Amylase

Trypsin

Lipase

Maltase, sucrase, lactase

Peptidase

Breaks down starches into disaccharides

Breaks down proteins into large peptides

Continues the breakdown of starch

Continues the breakdown of protein

Breaks down fat

Breaks down remaining disaccharides into monosaccharides

Breaks down dipeptides into amino acids.

Pancreas and Liver

• As chyme enters duodenum of small intestine, add from:

• Pancreas– For digestion:

• Amylase – continued starch breakdown• Trypsin – protein; • Lipase – fat breakdown• Sodium bicarbonate – neutralizes stomach acid

– Also: Enzymes that regulate blood sugar levels.• Liver

– Bile – acts like a detergent • Disperses fat into droplets• Lipase then breaks down fats• Gall bladder – Bile storage area

The liver and pancreas

Liver

Gallbladder

Duodenum

Bile duct

Pancreas

Pancreatic duct

To small intestine

Section 38-2

Small Intestine

• Three parts– Duodenum – very short (< 1 meter)– Jejunum (3 meters)– Ileum (3 meters)

• Role: Absorption of nutrients– Folds - Villi Microvilli

• Contain capillary fields• Chyme moved slowly across surface• Absorb protein and carbohydrate products

– Lacteals• Lymph vessels which absorb fatty acids and undigested fats

Small Intestine Villus

Circular folds

Villi

Epithelial cells

Capillaries

Lacteal

VeinArtery

Large Intestine

• What remains?– Cellulose and undigested materials– Water– Bacteria

• Purpose: Water absorption– Some vitamins produced here (K)

• Defecation– Elimination of remaining waste material through

rectum. (Solid excretion)

Key Functions of Digestion

• Mastication (Mechanical)• Salivation (Chemical)• Peristalsis (Mechanical)• Actions of Enzymes/Acid (Chemical)• Muscular Digestion (Mechanical)• Absorption • Defecation